Thermoforming is a known technique by which a heated plastic foil can be shaped using a mold assembly. Generally, the mold assembly comprises two molds that can be moved relative to each other for clamping the heated plastic foil. At least one of these molds is provided with one or more forming cavities. The other mold is provided, for each forming cavity, with a pre-stretcher that can be moved into the forming cavity thereby stretching the plastic foil. Thereafter, an overpressure will be generated on one side of the plastic foil to force the plastic foil against the wall of the forming cavity. There, the plastic foil cools down and the shape of the product is defined.
For some applications, wall thickness of the final product, both in terms of size and variation among different products, is an important parameter. For instance, the thermoformed product may be required to cooperate with another product or device, wherein the latter product or device imposes size restraints on the thermoformed product. Typically, the size restraints only apply to certain parts of the product. As an example, a particular region of the bottom wall of the product may need to have a thickness within a predefined range.
DE 10 2007 023 301 A1 presents a solution to address this problem. In this known system, an upper mold of the mold assembly of the thermoforming apparatus is provided with a calibration element in the form of a pen that is either fixedly attached to the upper mold or it is moveably attached thereto. In this known system, the pen is partially arranged in the pre-stretcher. During operation, the pen is moved downward to engage the plastic foil before pre-stretching the plastic foil using the pre-stretcher. By clamping the plastic foil in between the pen and a wall of the forming cavity, it becomes possible to accurately define the wall thickness of the product at the position where the plastic foil is clamped.
There is a continuing demand for increasing the throughput of thermoforming apparatuses while at the same time allowing smaller variations in product geometry. The applicant has found that although the abovementioned known system provides a solution for products having a particular geometry, it may still show wall thickness variations for some product geometries that may be unacceptable for some applications.
It is an object of the present invention to provide a solution for this problem.